New Mexico Geological Society Annual Spring Meeting — Abstracts

The Hermosa mining district is situated among the eastern foothills of the Black Range in southwest New Mexico and comprises a block of welI-faulted ground between two major tectonic features; 1) the 35 m.y. old Emory Cauldron, and 2) the Winston Graben. Exposures are dominated by a thick sequence of Paleozoic sediments which range from Ordovician through Permian in age and are flanked on the west by Tertiary volcanic and volcaniclastic rocks of the Mogollon-Datil field. Fully 95 percent of the ores have been mined from Upper Ordovician and Silurian dolomites beneath an impermeable and non-reactive sequence of Devonian siltstone and shale.

Normal faults of two general trends (north to northwest and east-northeast) form the structural pattern. Three major faults belonging to the first group transect the mining district, are downthrown to the west, and structurally control small plugs and dikes of flow-banded rhyolite. They are interpreted as elements of a wide ring fracture zone which marks the northeast periphery of the Emory Cauldron. Mineralization occurs between ring fracture faults in areas of complex dislocation that feature many sub-parallel, small-displacement breaks and their attendant fracture.

The deposits occur as thin stringers, small pods and pockets, irregular nodules and disseminated grains of sulfides associated with talc and variable amounts of calcite and quartz in laterally-persistent fissure veins. The ores consist of galena and sphalerite, typically in coarse-grained aggregates, chalcopyrite and fine-grained silver-bearing minerals including acanthite, tetrahedrite, polybasite, pyrargyrite and stephanite. Fluid inclusion thermometry studies indicate that the are deposits precipitated between 240°C and 350°C (pressure corrected) from weak to moderately saline brines (2.1 to 11.4 equivalent weight percent NaCl).